Generally, work vehicles used in the industrial and nonindustrial field, such as for example fork-lift trucks, trucks, earth-moving machines, et cetera, are provided with service brakes or parking brakes actuated by elastic means which allow to slow down the moving vehicle and keep it stationary in safe conditions.
These service brakes are generally connected to a hydraulic system, which is provided with jacks adapted to contrast the force applied by the elastic means.
The branches for feeding the jacks are connected selectively to the pressurized delivery branch or to the discharge branch of the hydraulic system by means of a control valve, which is adapted to control, by way of the gradual opening/closing of the corresponding connecting ports, the pressure of the liquid (normally but not exclusively oil) that acts on the jacks.
By modulating the pressure of the liquid that acts on the jacks, the resulting braking force that acts on the brakes is adjusted and accordingly so is the intensity of the braking force.
The control valves used in these hydraulic systems have manual activation, i.e., are actuated directly by an operator by means of a lever, a pedal or the like.
If a malfunction of the hydraulic system, of the engine system or the like occurs, with a consequent drop in pressure on the delivery branch, once the service brakes have been activated the work vehicle cannot move, since the lack of pressure in the delivery branch prevents the overcoming of the force applied by the elastic means and therefore prevents the release of the brakes.
In emergency conditions, such as when the work vehicle is a particular hindrance or danger in performing normal work or traffic operations, it may be necessary to remove such vehicle.
To allow this movement, the hydraulic systems can be provided with safety means for emergency deactivation of the service brakes.
These safety means are distinguished according to the manner in which they deactivate the braking system, i.e., by mechanical or hydraulic action.
The former in practice apply a direct mechanical action to the jacks in contrast with the force applied by the elastic means, compressing them and thus reducing the braking force.
However, these mechanical means require the operator to leave the control cab of the work vehicle and act manually on the jacks. This operation is not only awkward but also unsafe, since the operator is close to the vehicle when the service brakes are disabled.
As an alternative to mechanical safety means, hydraulic means are used.
Hydraulic safety means use an emergency cock with manual activation, which is arranged along the branch for feeding the jacks downward of the control valve and which, in normal operating conditions, connects the feed branch to such valve, but in emergency conditions can be switched in order to connect the feed branch to the discharge branch or to another auxiliary duct.
In this manner, a quantity of liquid drawn from the discharge branch or from the auxiliary duct and sufficient to allow the momentary or prolonged deactivation of the service brakes is sent into the active chamber of the jacks by way of pumping means.
These known hydraulic safety means are not free from drawbacks, which include the fact that by switching the emergency cock to the position that connects the feed branch to the discharge branch or to the auxiliary duct, in practice the delivery branch is disabled.
In this position, by bypassing the control valve, the jack feed branch is in fact no longer connected to the delivery branch, and therefore it is not possible to restore the normal operation of the hydraulic system, and therefore of the braking system, without first returning the cock to the initial position.
To restore the normal operation of the braking system, it is accordingly necessary to perform at least two operations, i.e., switch the emergency cock to the initial position and act on the activation lever of the control valve in order to return it to the desired configuration.
These operations are awkward and do not allow immediate use of the work vehicle after solving the problems that caused the malfunction.